Most of the time he is in what is known as a chibi form. It is said he can only revert back to Normal Tanaka for a few minutes before reverting back to his small form.
It seems Tanaka served Ciel's parents as well. When Lord and Lady Phantomhive were murdered, the young Ciel ran to Tanaka, who warned him away. Tanaka was then stabbed by an unseen assailant...